The problems I will investigate have to do with the importance of physical contrast in vision. Contrast and color seem to be analyzed in parallel neural subsystems in the primate brain. The first problem is to find out how parallel neural pathways, composed of color-selective and contrast-sensitive neurons, are elaborated in the visual systems of primates. This problem will be studied by measuring the gain and contrast gain as functions of mean level in neurons of the monkey lateral geniculate nucleus (LGN). I also propose to complete an analogous study in the cat's visual pathway. Using the cat as a model system, I have been able to develop and to validate the concepts which relate contrast sensitivity to light adaptation. The experimental results of these investigations will allow me to test models for retinal gain control mechanisms. I also propose to study contrast gain as a function of retinal eccentricity. To avoid the variability created by pooling data from different animals, I propose to sample large populations of LGN and cortical units from individual animals and to record from neurons representing the center and periphery of the visual field simultaneously with different microelectrodes. Another specific problem is to determine mechanisms of intracortical interactions: the cortical contrast gain control, cross-oriengation suppression, and spatial-frequency inhibition. I will do this by measuring the gain and phase of the response to drifting grating patterns in the presence of conditioning stimuli. We will also measure the magnitude and extent of visual assimilation observable in neurons in the retina, LGN, and visual cortex. Together with my colleagues, I have devised visual patterns which may be used to measure assimilation independent of contrast. The strength of assimilation compared to the neural response to contrast is a crucial feature of theories of perceptual brightness, for example Land's retinex theory.